IV & V
16 20 25 30 35 40+- 16 20 25 30 35 40+ 16 20 25 30 35 40+ Mother's age
Figure 4. Variations in postneonatal mortality ratios with birth rank of infant (= mother's parity) and mother's age in different social classes, England and Wales, 1949-1950.
Source: Morrison et al. (27).
Evidence from the State of New York that is comparable in many respects to that of Morris and his associates has been provided by Chase (29, 30). She studied "nearly one-half million births" which occurred in New York State, exclusive of New York City, in the years 1950-52. The major study group consisted of single, white fetal deaths and live births. Through examination of death certificates, information was obtained for each child who died within 5 years of birth, and this was related to information from the birth certificate of the same individual. Her data concerning the postneonatal and early childhood deaths are relevant here since environmental factors may be expected to be most important after the neonatal period. The variations in mortality rates by birth rank are shown in Table 4, in which the increase associated with family size is obvious.
Analyses of her data had indicated that increasing mortality rates were associated with prematurity, maternal age, and social class. In her second paper (30) she controlled for prematurity and maternal age by examining the mortality in children whose mothers were 20 to 29 years at the time of their births and whose birth weights were between 2,501 and 3,500 grams. Theses—social class, maternal age, and family size—is controlled. When rates are compared, the findings shown in Figure 3 were obtained. Mortality ratios bring out the differences more dramatically, as is evident in Figure 4 (27). Mortality rates increase with family size in all social classes, but the effects are most powerful in the younger mothers, regardless of social class. (A young mother with a large family will, of course, have closely spaced children. Birth interval will be discussed later.) The authors comment: